Stabilization of wood and wood products with styrene acrylonitrile, bis(2-chloroethyl)vinyl phosphonate



Feb. 12, 1963 D l. KEAGA 3,077,418

STABILIZATION OF WOOD` AD WOOD PRODUCTS WITH STYRENE ACRYLONITRILE BIS( 2CHLOROETHYL )VINYL PHOSPHONATE Filed Feb. 5. 1958 Wo oc/ 5w@ fng Swe//l'ng Gyver??l /rrad/'a /fon /rraa//a//on source n?. 2 OUOnQLA/enaya 19 fOAA/fy This invention relates to stabilization of wood and natural and artically formed lignocellulose-containing materials and more particularlyl to a new and useful method for the stabilization of wood and wood-like prod-V Vucts against dimensional change and to compositions for effecting such stabilization. l

It is well known that materials made up entirely or predominantly of cellulose iibers expand and contract with variations in humidity in the ambient atmosphere.

In wood, for example, such swelling and shrinking isV accompanied by warping and checkin-g. These properties are particularly undesirable when wood is used as a structural material. Wood undergoes an increase in dimension upon absorption of moisture from the atmosphere and a contraction when moisture is given up to the atmosphere. In a very humid atmosphere swelling continues until a moisture content of between 28 and 30 percent is reached. This moisture content is known as the liber-saturation point. Below the fiber-saturation point the water enters the tine capillary structure of the cell walls and affects the dimensions of the wood. Above this moisture content the water enters the cell cavities or the coarse capillary structure. The water present in the coarse capillary structure of the wood is called free water and does not affect the swelling and shrinking under normal conditions.

Stabilization of wood against dimensional changes has been attempted heretofore with only limited success. Some of the more important general methods which have been employed are (l) coatings, both external and internal; (2) deposition of bulking agents in the cell walls, (3) decreased hygroscopicity plus bulking; and, (4) chemicaliy cross-linking the cellulose molecules, thereby reducing the degree to which wood` can swell. An example of the cross-linking method of dimensional stabilization is the treatment of wood with formaldehyde. The stabilization is obtained by a reaction between formaldehyde and the hydroxyl groups of the cellulose molecules connectin g together the latter by cross-bridges of acetal linkages. @ne of the principal disadvantages of the formaldehyde treatment is that it requires anhydrous conditions `and very corrosive mineral acids as catalysts, resulting in extensive hydrolytic degradation of the wood. An example of `the bulking technique for the stabilization of wood is the impregnation of the swollen wood with a thermosetting resin. In this method, an unpolymerized resin is permitted to diffuse into the cell walls andthe treated wood heated to polymerize the resin within the cell walls to form an irreversible polymerized compound which holds the wood in a swollen state. This treatment, however, changes the physical properties of wood, notably its resiliency and shock resistance. Furthermore, resin treated wood is very heavy and very hard. The resin process has other disadvantages from the treatment standpoint in that it is only commercially practicable for veneer or plywood. A more recent theory under which wood is stabilized is by reduction of hygroscopicity plus bulking. The theoretical principle is thought to be a replacement of some of the hydroxyl groups of the cellulose structure by reaction with the treating material plus bulking the cell w-alls by the specific volume of the treating material. An example of this technique is acetylation with acetic arent aqueous or organic solvent solutions.

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anhydride and catalysts and swelling agents such aspyri.- dine or. dimethylformamide. This procedure is not conimercially practical for large cross section treatment.

lt is, therefore, an object of this invention to provide, a new and novel method for the dimensional stabilization o f wood and other lignocellulosic materials. A further` object is to provide a method for reducing the shrinkage and swelling of wood. A still further object is to provide wood resistant to warping and checking. A still further object is to provide a dirnensionallystabilized wood which when stabilized is free of the degradation caused by using known cross-linking agents and catalysts. These and other objects will become evident from the following specification and claims.

FlGURE 1 represents a schematic flow diagram of the process described hereinafter for treating lignocellulosio materials with a styrene, acrylonitrile, and/or bis(2 chloroethyDvinyl phosphonate impregnant;

` FIGURE 2 is a schematic diagram of an alternative method for treating lignocellulosic materials in accordance with the present invention to accomplish dimensional stabilization of the material.

In accordance with the present invention, it has been found that effective dimensional stabilization of 4any wood` or other lignocellulosic material can be obtained by treat` ment thereof with a non-leachable, non-swellable, insolubilizable compound having radiation-activ-atable reactive groups and irradiating the so treated wood under conditions whereby a significant proportion of the treating compound is retained in the wood. The resulting wood product, containing from 10 to 50 percent, or more, by weight, of the compound in the form of a non-leach# able, non-swellable water insoluble product associated with the wood, is found to be stabilized against dimensional changes, warping and checking resulting from effects of moisture. Although the amount of compound desirable in the wood structure will vary with the intended use of the stabilized material, the presence of 15 to 30 percent by weight of the compound is considered preferable for most purposes.

By employing the treatment of the present invention, wood may be dimensionally stabilized or made resistant to swelling and shrinking without altering greatly its physicalproperties. Furthermore, `anhydrous conditions are not'required in carrying out this process. The present treating compounds may be Vintroduced into the wood from substantially any wood-swelling solvent, suchV as The subsequent irradiation provides a degree of stabilization orreduction in `swelling of from about l0 to 70 percent or more. The

percent reduction in swelling depends in part von the weight of the compounds retained by the wood. The process of the present invention is applicable to commercial application where relatively large cross-sections` are to be stabilized.

In accordance with this invention, wood is immersed in the impregnate or in the impregnating compound dissolved or dispersed in a wood swelling agent to assist in the penetration of the former into the cell wall and into intimate contact with the cellulosic structure. Alternatively, the wood may be swelled with a wood-swelling agent and then immersed in the impregnant or a. solution of the impregnant. This immersion or impregnation step may b e .conducted at atmospheric pressure or at ,sub-or Vsuper-at,- mospheric pressure, and at room temperature or slightly `elevated temperatures. Such impregnation methods are well known to those versed in the art of wood preservation. It is generally preferred to operate at a temperature range of about from 15 to 85 C. Such temperatures give good results, although somewhat higher and lower'V .temperatures may be` employed without materially altecting theresults The treating compound may he applied from aqueous solution, but solutions in organic solvents may also be employed with equal success when good recovery permits reuse of the solvents. In some cases the treating compound may also serve as the swelling acrylonitrile mixtures and styrene-2|.crylonitrile-bis(Z-chlo-l roethyl)vinyl phosphonate mixtures.

`The concentration of impregnating or treating solution will depend in part on the specilic treating compound, the solvent, the temperature, and the type of irradiation. Generally, a l5-30 percent solution has been found to be convenient,` although any suitable concentration may be employed. V', The preferred conditions for treatment Will depend on the particular wood or wood product, the dimensions thereof, the treating compound, the type and degree of irradiation, the solvent, and, in some instances other factors. After an immersion period in the treating solution, the impregnated wood is curedl by exposure to ionizing radiation for a suitable period of time to bring about a reaction, bonding or other association between the treating compound and the cellulosic material so that a significant proportion ofthe treating compound is retained within the ywood '(lignocellulosic) structure. In selecting a Isuitable radiation source, high-energy particulate radiation or high-energy electromagnetic radiation may be employed. Thus, one may employ atomic particles, neutrons, photons, gamma rays, X-rays, electrons, deuterons, and ssion fragments from nuclear reactors or accelerators, or from articial or natural radioactive isotopes. Good results have been obtained by irradiation from sources emitting from 102 to 10rl rads./sec. such as a 2 to remove unreacted volatile solvents and/or unreacted or solvent soluble compounds, and in laboratory comparison techniques leached with running distilled water until the pH of the leached water is the same as that of the untreated concurrently leached controls to remove any unassociated compound or water soluble solvents.

For the purposes of comparison and to determine the degree of dimension stability, the leached Wood pieces are oven dried at 10U-105 C., measured along the tangential direction, re-weighed, soaked in water, and re-measured. From the difference in the dried weights before and after treatment with the compounds, the percent compound in the treated wood can be determined. From the difference in tangential dimensions of the treated soaked Wood and the treated oven-dried wood, the percent swell- .i

ing of the treated wood can be determined. Similar measurements are made on controls of untreated Wood. From the data, the degree of dimensional stabilization or reduction in swelling can be calculated according to the following equation:

Percent reduction in swelling (Percent swelling of untreated wood) minus (Percent swelling of treated wood) Percent swelling of untreated wood The following examples are illustrative of the present invention but are not to be construed as limiting the same.

Example 1 Y were immersed in dimethylformamide containing 20 percent of a 50-50 molal mixture of styrene andk acrylonimev.'(million electron volts) source at beam currents up -i to.14.5 microamps. Good results have also been obtained by irradiation from a cobalt 60 source at dose rates'up to 650,000 rads./hr. (698,750 reps/hn). Wood or wood products may be irradiated up to a dose of l0I rads. Irradiation at ldoses greater than 107 'rads. will probably cause severe degradation of the cellulosic structure.

' After the curing step, the unassociated treating agent and swelling agent may be removed from the wood. The result of this series of operations is that a significant residue ofthe stabilizing compound is reacted with or in some manner associated with and retained in the wood structure, imparting there-to the property of resistance to dimensional change `resulting from variations in ambient humidity.

onepr'ocedure for treating wood to obtain dimen- Isional stabilization, an air-dried ponderosa pine sapwood block, for example, is weighed, and accurately measured inthe tangential direction. The wood is then impregnated with one of the treating compounds by vacuum impregna- `tion at an appropriate temperature and pressure. In many instances the impregnate is a solution of the compound dissolved or dispersed in a wood-swelling agent, such as dimethylformamide. Subsequently, the block is removed from the solution and may be sealed in a waterand air-impervious, plastic material to prevent loss of treating solution. The so packaged wood is irradiated to from 1,02 to 107 rads. by, for example, a 2 million-voltelectron beam (Van de Graaff generator) with a current of from 5.6 to 56 microamps. with the wood in the electron beam about 20 percent of the time. In other instances, the impregnated wood is irradiated in a cobalt 60 source at a dose rate of about 156,000 rads. per hour. Following the irradiation, the Wood, if sealed in the plastic bag, is removed from the bag and conditioned in air trile by subjecting the wood blocks iirst to a Vacuum of l0 mm. of mercury for ten minutes and then introducing into the evacuated chamber the treating solution. During this operation the pressure was restored to atmospheric pressure. The blocks were held immersed for about l0 minutes and then the wafers were 'packaged in small plastic polyethylene bags to prevent loss of treating solutions. The so packaged wafers and untreated checks were subjected to electron irradiation from a Van de Graaff electrostatic generator. Following the irradiation, the wafers were removed from the polyethylene bags, conditioned to remove any unreacted volatile solvents, monomers, or compounds and then leached in running distilled water for three days. The blocks were then carefully dried to below fiber saturation point to prevent checking and then oven-dried at C. for 24 hours. The wafers were then measured and weighed at the oven-dried conditions to determine retention and the tangential dimensions, and then were re-swelled in water to obtain the swollen dimensions. The resul'tsof measurements taken before and after treatment compared to the controls are recorded below as percent reduction in swelling.

Example 2 In a like manner as above, employing dimethylformamide containing 20 percent by weight of a mixture of styrene, acrylonitrile and bis(2chloroethyl)vinyl phossprayers phonate in mole ratios of1:1:1;2:2:1;4:4:1and12110, respectively, the following results were obtained:

What is claimed is:

1. A process for the dimensional stabilization of ligne cellulosic materials which comprises the steps of swelling the said lignocellulosic material with an organic swelling agent, while simultaneously impregnating the said so swelled material with a mixture or styrene, acrylonitrile and bis(2chloroethyl)viny1 phosphonate, consisting of a mixture of about one mole of acrylonitrile and about zero to one mole of phosplionate per mole of styrene, and irradiating the so treated Wood with from 102 to 10' rads of high energy radiation for from 1 sec. to 24 hours.

2. A process for the dimensional stabilization of lignocellulosic materials as set forth in claim 1 wherein the molar ratio of impregnate components is 1:1:0 respectively.

3. A process for the dimensional stabilization of lignocellulosic materials as set forth in claim 1 wherein said impregnate components are mixed in a molar ratio of 1:1:1/2 respectively.

4. A process for the dimensional stabilization of lignecellulosic materials as set forth in claim 1 wherein said impregnate components are mixed in a molar ratio of 1:1:14 respectively.

5. A process for the dimensional stabilization of 1ignocellulosic material which comprises the steps of swelling the said lignocellulosic material with an organic swelling agent, impregnating the said material with a mixture of styrene, acrylonitrile and bis(2ch1oroethyl)vinyl phosphonate consisting of about one mole ot acrylonitrile and about zero to about one mole of phosphonate per mole of styrene respectively, and irradiating the said so treated material with from 102 to 107 rads of high energy particulate radiation for from 1 sec. to 24 hours.

6. A process for the dimensional stabilization o lignocellulosic materials which comprises the steps of swelling the said lignocellulosic material with an organic swelling agent, impregnating the said material with a mixture of styrene, acrylonitrile and (2-chloroethyl)viny1 phosphonate consisting of about one mole of acrylonitrile and about zero to about one mole or" phosphonate per mole of styrene respectively, and irradiating the said so treated material with from 102 to 107 rads of high energy magnetic radiation for from 1 sec. to 24 hours.

7. A process for the dimensional stabilization of wood which comprises the steps impregnating the wood with a mixture of styrene, acrylonitrile and bis(2-chloroethyl) vinyl phosphonate, said compounds being mixed in a ratio consisting of about one mole of acrylonitrile to about zero to one mole of phosphonate per mole of styrene and irradiating the so treated wood with from 102 to 107 rads of high energy radiation for from 1 sec. to 24 hours.

References Cited in the file of this patent UNITED STATES PATENTS 1,241,738 Klatte Oct. 2, 1917 2,186,360 Britton `lan. 9, 1940 2,352,740 Shannon luly 4, 1944 2,636,027 Coover Apr. 21, 1953 2,670,483 Brophy Mar. 2, 1954 2,790,736 McLaughlin Apr. 30, 1957 2,793,970 Jeppson May 28, 1957 OTHER REFERENCES BNL 389 (T-73), Progress Report on Pission Products Utilization Vil, Brookhaven National Lab., May 1946, especially page 19.

Sun: Modern Plastics, vol. 32, No. 1, September 1954, pp. 141-144, 146-148, 150, 229-233 and 236-238. 

1. A PROCESS FOR THE DIMENSIONAL STABILIZATION OF LIGNOCELLULOSIC MATERIALS WHICH COMPRISES THE STEPS OF SWELLING THE SAID LIGNOCELLULOSIC MATERIAL WITH AN ORGANIC SWELLING AGENT, WHILE SIMULTANEOUSLY IMPREGNATING THE SAID SO SWELLED MATERIAL WITH A MIXTURE OF STYRENE, ACRYLONITRILE AND BIS(2-CHLOROETHYL)VINYL PHOSPHATE, CONSISTING OF A MIXTURE OF ABOUT ONE MOLE OF ACRYLONITRILE AND ABOUT ZERO 